1. Field of the Invention:
The present invention relates to a signal receiver for use in a global positioning system (GPS), and more particularly to a GPS signal receiver suitable for use especially in regions which suffer multipath interference.
2. Description of the Related Art
One conventional GPS signal receiver is shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1, a signal transmitted from a GPS satellite (hereinafter referred to as a "GPS signal") is received by an antenna 1 on a mobile body such as an automobile, amplified by an RF amplifier 2, and converted into an intermediate-frequency signal by a frequency converter 3 for easy subsequent signal processing. Since the GPS signal has been modulated by a PN (pseudorandom noise) code so as to be spectrum-spread, it is necessary that the received GPS signal be correlated with a locally generated PN code so as to be code-stripped. To meet such a requirement, the GPS signal as converted into the intermediate-frequency signal is supplied to a PN code correlator 5 which also is supplied with a PN code generated by a PN code generator 4, and correlated with the PN code from the PN code generator 4 so as to be code-stripped by the PN code correlator 5 (see, for example, GLOBAL POSITIONING SYSTEM (Papers published in NAVIGATION), P.M. Janiczek, Editor, The Institute of Navigation, 1980, pp 51-53, Spilker: "Signal Structure and Performance Characteristics")
An output signal from the PN code correlator 5 is supplied selectively to a signal search unit 7 and a signal tracking unit 9 through a switch 6A. Output signals from the signal search unit 7 and the signal tracking unit 9 are supplied to a PN code phase control unit 10A, which produces an output signal to control the phase of a PN code generated by the PN code generator 4.
After the power supply of the conventional GPS signal receiver is switched on, the switch 6A is shifted over to the signal search unit 7. Until the PN code of the received GPS signal as converted into the intermediate-frequency signal is brought into phase with the PN code generated by the PN code generator 4, the phase of the PN code generated by the PN code generator 4 is successively varied to various phases in response to the output signal from the signal search unit 7 under the control of the PN code phase control unit 10A. At this time, the correlated output signal from the PN code correlator 5 is monitored by the signal search unit 7 which searches for a point where the correlated output signal exceeds a predetermined value.
When the correlated output signal exceeds the predetermined value, the switch 6A is shifted over to the signal tracking unit 9, and the correlated output signal from the PN code correlator 5 is monitored by the signal tracking unit 9. The phase of the PN code produced by the PN code generator 4 is controlled by the PN code phase control unit 10A such that the correlated output signal will remain at its peak value.
When the PN code of the received GPS signal is out of phase with the PN code generated by the PN code generator 4, the correlated output signal is low. Conversely, when the PN code of the received GPS signal is in phase with the PN code generated by the PN code generator 4, the correlated output signal is high, as shown in FIG. 2 of the accompanying drawings. The correlated output signal is substantially eliminated if the PN codes are shifted out of phase with each other by one bit. In the conventional GPS signal receiver, therefore, the GPS signal is tracked to keep the peak value of the correlated output signal which has been searched for by the signal search unit 7.
However, the conventional GPS signal receiver has suffered the following disadvantages: When the GPS signal receiver is installed on an automobile and moved through an environment or region being subjected to multipath interference, such as an urban district, there is caused a differential delay between a direct wave and a multipath wave. At this time, if the correlated output signal searched for by the signal search unit 7 is produced by the multipath wave, then the measured position that is obtained by a GPS associated with the GPS signal receiver tends to deviate from an actual position. Furthermore, as the automobile moves, the multipath around the automobile varies from time to time, and hence the measured position suffers various deviations or errors. Consequently, the conventional GPS signal receiver fails to give accurate information which would enable the GPS to produce accurate measured positions (see, for example, ibid, pp 8-11, Milliken & Zoller: "Principle of Operation of NAVSTAR and System Characteristics", p 36, Spilker: "Signal Structure and Performance Characteristics", p 86, Glazer: "GPS Receiver Operation" and pp 115-117, Martin: "User Equipment Error Models").